1. Field of the Invention
The present invention relates generally to synthetic organic chemistry and plant growth hormones. More particularly, it concerns the efficient preparation of gibberellin A5 in a cost-effective manner.
2. Description of the Related Art
There are six major groups of plant hormones: auxins, gibberellins, ethylene, cytokinins, brassinosteroids and abscisic acid. The more commonly used groups in agriculture, floriculture, forestry and horticulture are the auxins, gibberellins, ethylene and cytokinins. Gibberellins are a group of tetracyclic diterpenoid compounds, and the skeleton below shows the carbon numbering of the C19 class of gibberellins:

Many gibberellins occur naturally in plants and some of these gibberellins can be produced by fermentation methods. There are over 130 gibberellins discussed in the literature. Individual gibberellins are designated with an integer that reflects the chronological order of discovery. Four examples of gibberellins that occur naturally in higher plants are shown below and three of these (gibberellins A3, A4 and A7) can also be produced in commercial quantities by fungal fermentation.

Naturally occurring gibberellins are used in all of the above-mentioned fields to, for example, promote flowering. Three gibberellins have been used extensively for commercial purposes: GA3 (also known as gibberellic acid) and a mixture of GA4 and GA7 (GA4/7). All three are native to higher plants and can be derived from fungal fermentations. However, when applied exogenously to higher plants they can often have detrimental side effects. For example, GA3 often produces excessive stem elongation when applied to many plants, which can be undesirable in terms of lodging (falling over) in rain, or aesthetically when used for floriculture purposes. Like GA3, GA4/7 can also promote excessive (undesirable) stem elongation and both GA3 and GA7 can be excessively persistent when applied to higher plants due to the C-1,2 double bond in Ring A (shown above). Therefore, while applied gibberellins can promote flowering, there may also be the detrimental effect of increased shoot and stem growth (excessive elongation).
Preventing excessive shoot growth in a flowering plant is useful in many circumstances. For example, this effect makes the plant more resistant to falling over under adverse weather conditions such as wind, rain, hail and snow. This effect also makes the plant more compact, more stocky and more resistant to lodging as a result of weather conditions or as a result of heavy fruit or seed or grain production. In orchard situations, or in floriculture, for example, a more compact nature of a shrub or tree is valuable for a variety of reasons, including ease of tending the plant, picking the fruit, aesthetics, applying other treatments and reducing the necessity to prune the plant.
For the promotion of flowering and other benefits, the use of GA5 offers advantages over the currently used GA3. For example, GA5 is an exceptionally florigenic molecule (King and Evans, 2003), but does not suffer the side effects of excessive shoot growth and flower stalk elongation to the degree seen with GA3 and other gibberellins (King and Evans, 2003; Pharis, 1972; Ben-Tal, 2000). Moreover, the use of an inexpensively produced GA5 preparation offers cost advantages over, for example, GA4/7. GA5 has been prepared by some groups; however the processes are frequently inefficient and the reagents quite expensive. Since it has been very expensive to synthesize, GA5 is typically not used in agriculture or the other commercial areas mentioned above.